This invention relates to the fabrication of shaped articles of cellular organic resins by heating expandable polymer compositions in molds.
It is well known in the art to make cellular resin products from organic polymer compositions containing blowing agents, and materials, compositions and procedures suitable for the purpose have been described. Techniques have been devised for fabricating such expandable compositions into useful articles having resulting cellular structure. Among such techniques are molding by extrusion of foaming heated masses through shaping die orifices, molding by injection of hot foaming material into closed molds, and compression molding of foamable material in closed molds by heating under pressure followed by release of pressure. Some of these techniques, suc as extrusion, are suitable for making simple shapes such as sheet, plank, cylinders and tubes, but are not suitable for making more intricate, three dimensional shapes.
Of interest in making shaped articles which are intricate in shape and large in size are those techniques called chest or autoclave molding wherein beads of expandable thermoplastic organic polymer material are charged to a mold that is permeable to fluids and wherein steam or hot air is blown into the mold to heat and cause the beads to soften, expand to fill the mold, and fuse together. Variations of such molding have been suggested wherein a permeable mold, charged with expandable thermoplastic resin beads, is submerged in a bath of hot liquid, e.g. hot water or other liquids such as glycerol and aqueous salt solutions, as heating medium. For example, Stastny et al. in U.S. Pat. No. 3,823,213 describe production of moldings of expanded polymers of certain crosslinked olefin polymers in which previously expanded particles of the polymer are confined and heated in closed molds perforated to allow fluid heating medium to enter and gases to escape. Fluid heating media which are mentioned are hot gases or vapors, e.g. hot steam and mixtures of air and steam, or liquids, examples of which are water, glycerol or aqueous (water) solutions of salts or of high boiling liquids which are miscible with water.
Chest molding techniques have conventionally been applied to relatively low softening temperature plastic compositions with volatile liquid blowing agents, such as beads of polystyrene containing pentane. Greater difficulties are encountered when the expansion requires higher temperatures and the use, for example, of higher pressure steam. While high boiling organic liquids can provide higher temperature without use of pressure, they generally have relatively low heat capacity and low heat transfer properties and can adversely affect or contaminate the plastic being molded.
It is an object of this invention to provide improvement in fabrication of shaped articles of cellular thermoplastic resins by heating expandable polymer materials in molds.
A more specific object is to provide said improvement in heating expandable polymer materials in molds permeable to liquid by providing improvement in the liquid heating means, particularly for use at relatively high temperatures.
Other objects and advantages of the invention will become evident from the description that follows.